The invention relates to the field of pistons for internal combustion engines, particularly for motor vehicles, heavy goods vehicles, agricultural machines, public works machines, and ships.
In recent years high-performance internal combustion engines have been developed which in particular have higher levels of specific power in order to meet new and future anti-pollution standards on CO2 emissions. This is particularly true in the case of diesel engines. This increase in the specific power levels involves a very substantial increase in the thermal and mechanical stresses to which the engine parts, and particularly the pistons, are subjected. Consequently the design of pistons is becoming increasingly complex.
Pistons are usually produced in one piece from moulded or forged aluminium alloy. However, the increased stress conditions which have just been mentioned render the conventional pistons unsuitable. Consequently various solutions have been conceived to render the aluminium pistons compatible with the high-performance engines: insertion of alumina fibers in the alloy to reinforce it, addition of steel inserts to reduce the expansion, deposition of graphite on the skirt to reduce friction, or machining of cooling channels to make the air or oil circulate there in such a way as to keep the piston at acceptable operating temperatures. However, all these solutions are expensive.
One conceivable solution might be the replacement of the aluminium alloy by a steel which, with comparable geometry, would have a better resistance to the mechanical and thermal stresses and to fatigue and a better temperature resistance. In fact, in the past steel has been used to manufacture pistons, but the use of steel for the manufacture of pistons for high-performance engines is not in fact conceivable first and foremost from the point of view of economics, because of the high density of this material. If it were desired to give the piston a sufficiently low mass in order to obtain high performance of the engine, it would be necessary to arrive at a very reduced wall thickness after forging of the piston. Such a thickness is inaccessible using conventional forging techniques if, for reasons of cost, it is desired to continue producing pistons in one piece.
The object of the invention is to render possible the manufacture, under economically advantageous conditions, of pistons for high-performance internal combustion engines, particularly making it possible for this purpose to use a steel, or another dense alloy with high mechanical properties, instead of a specially treated and/or shaped aluminium alloy.